The present invention relates to an image correction system for a scanning electron microscope (hereinafter referred to as SEM in abbreviation). More particularly, the present invention concerns an image correction system which can be advantageously used for correcting disturbances appearing in a SEM image under the influence of vibration superposed on the scanning operation, reducing the blur of images by averaging image data over a plurality of image frames, and further correcting image distortion produced under action of the magnetic fields generated by power sources employed in peripheral equipment.
FIGS. 2A and 2B of the accompanying drawings are views for illustrating schematically how a SEM image undergoes disturbance when vibration is superposed on the scanning by an electron beam. More specifically, FIG. 2A shows a pattern of an object under examination and scanning lines. As will be seen, the individual scanning lines are not only dislocated irregularly in the horizontal direction but also vertically deviated at random. In an extreme case, there may occur such situation in which the scanning direction is changed over to the vertical direction. However, since the maximum frequency of the scan vibration is low when compared with the scanning frequency, the scanning in the linear pattern can be assured although displacement, expansion/contraction and tilting of the scanning lines may occur, as will be seen in FIG. 2A. An image of the object shown in FIG. 2A picked up through the electron beam scanning is illustrated in FIG. 2B. It will be seen that the peripheral edge of the image pattern is disturbed with fluctuation in the horizontal direction, while interruption of the image pattern appears in the vertical direction.
Such image disturbance caused by the vibratory scanning in a scanning image pick-up system is also experienced during topographical observations of the earth using an artificial satellite. An approach for solving this problem, is proposed in the image distortion correcting system, as disclosed, for example, in Japanese Patent Application Laid-Open No. 58574/1984 (JP-A-59-58574). According to this known proposal, vibration in the attitude of the satellite which provides a main factor for the scan vibration is measured by means of a high-precision attitude angle detector, wherein the deviation in the scanning position (scan dislocation) is determined on the basis of a geometrical model. Data of the scan dislocation thus obtained is utilized for rearranging the image data for performing the correction of the image distortion. In contrast, in the case of the SEM, the scan vibrations are attributable to many vibration sources such as fluctuation in the magnetic field and power supply sources, not to say of mechanical vibration. Such being the circumstances, a great difficulty has been encountered in determining the magnitude of positional deviation or dislocation in a SEM image with an acceptable accuracy on the basis of a scan deviation model which must be prepared by measuring all the causes for the scan vibration with adequate accuracy.
Next, problems in an image processing system for a SEM in which an averaging procedure of a plurality of image frames is adopted will be described by referring to FIG. 15 of the accompanying drawings which shows an arrangement of the image processing system. Referring to the figure, an image signal obtained from the scanning with an electron beam in the SEM 1 is supplied to an analogue-to-digital or A/D converter 2 to be converted to digital image data, which data is then combined with average image data read out from an image data memory 3 through simple addition or addition/averaging operation in an image averaging unit 4, the resultant data being then written in the memory 3. The scanning and averaging processing for a single image mentioned above is repeated a multiple number times ten during a period of one second. Through the procedure of repeating the image averaging processing, as mentioned above, the image read out from the image data memory 3 by an image data reader 5 can enjoy high definition or clearness on an image display 10, even when the image obtained through a single scan operation is degraded in S/N ratio. In the case of the SEM which adopts the so-called multiframe averaging system incorporating the image memory, various positional alignment can be started at an earlier point in the average repetition process, which thus allows measurements of semiconductor specimen or the like with a small quantity of electron irradiation.
However, should mechanical vibration be transmitted to the SEM equipment, the scanning electron beam is deviated from a predetermined position at random. Consequently, in the addition and averaging processing, the image data at relatively deviated or dislocated positions will be added and averaged, resulting in an output image which can not be freed from blur. The magnitude of the blur is of course in proportion to the vibration applied. It should be mentioned that the correction of blur attributable to the mechanical vibration has not be taken into consideration in the hitherto known system.